Congratulations Nikitachi!
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Several weeks ago I was debating whether or not to get spa day for the fertility massage and spent some time trying to find any information about how much a fertility massage increases the chance of twins, to help me decide if it might be worth buying it. I didn't find anything on that point so thanks GlazeyLady for your explanation!
However while trying to find some information about that, I did manage to stumble across this thread in the spa day subforum:
https://www.carls-sims-4-guide.com/forum/index.php?topic=27643.0I tried Playalot's tip from that thread in my practice file twice and got twins both times unlike the first two pregnancies where I had only had single children. That could have been coincidence - both practice sims had the fertile reward from mid first pregnancy - but I couldn't really spare any more time practicing so I decided I would just go with it in the real thing. Taking an extra few hours to get pregnant each time would be worth it if it increased my chance of twins.
I usually made my sims try for baby twice, take a brief break while I watched closely for any sign of pregnancy, and then try for baby again, usually twice more. They had quite a lot of multiple births.
If I count up all the times I've tried Playalot's tip so far (in practice, my challenge file, and unofficially continuing my game after stuffing up my attempt)... that comes to a total of 7 pregnancies with only 2 single births among them. The other pregnancies were 4 sets of twins and 1 set of triplets. So it looks like Playalot's method doesn't guarantee you multiples but does seem to give a better chance of them.
(There's 1 pregnancy from my real file that's not counted in that data because the game wouldn't let me try for baby multiple times - presumably because there was only one spot in the house at that point).
My sims both had the fertile reward trait too though I don't know how much difference that would have made. From what other people have been saying about the fertile trait, I suspect not much. I didn't end up buying spa day and did nothing else with regards to fertility other than the fertile reward trait and Playalot's tip.
Playalot suggested looking out for your sim appearing nauseous or having baby related thought bubbles. Personally I found it much easier to watch my sim's hunger need - it drains faster than usual when a sim is pregnant even if a pregnancy test hasn't been used yet.
I assumed that some of the other players would know about this since I found it on these very forums. It seemed quite unlikely to me that no other challenge players had read that thread in the years since Playalot posted in 2016. Though I guess I was wrong in that assumption, as surely someone would have mentioned it by now. SueDenim did mention extra woohoo-ing but it's not clear if that means trying for baby again after the sim is showing signs of pregnancy but before a pregnancy test.
Maybe the high number of multiple births I've had from using this tip were just random luck - 7 pregnancies isn't exactly a large sample size after all. But personally I'm convinced that it does have some effect. And while I'm probably the last person on these forums who still doesn't have city living yet, if you ever need/want twins without the fertility increasing lot trait, you could try this to improve your chances.
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As for raising the kids, most of what I did has been covered already (clingy toddlers, high chair naps, etc). There are only two points I'd add:
Teens only need a B grade when they were a child in order to start a part time job at level 2. Of course then they only start high school with a C and you have to get their grade up.
Personally I find the child social aspiration the fastest, followed by creative. So most of them got social, but some got creative so that I didn't have to micromanage two kids working on social at the same time. (After doing that once, I decided never again!)
I had my parents go to park during the first pregnancy to meet kids which they could then invite over for my children working on the third milestone of the social aspiration. (And also so that my sims could complete "read to a child" from the family aspirations without having to wait for my own kids to be born/grow up). Once a parent invited over a child they knew, I just had my kids take photos with the visitor until they were friends & send them home again. The last child or adult to be befriended this way was usually kept on the lot for my kid to spam friendly socials until their social skill reached level 10. It always finished faster than creative for me - even with moodlet solvers and inspired potions, the creative kids would be less than half way through milestone 3 when their social twin completed their aspiration.
I almost ran into trouble when those first 3 townie children all aged up to teen around the same time but luckily the parents managed to meet one new child at the park while my kids at school managed to meet 2 more, so it didn't end up being an issue. That's said, if I did it again I would have probably go with making all children do creative at the 2 week point when I know the townie children are likely to all age up around the same time. Then only go back to social once I'd found 3 townie kids again.